1. Field of the Invention
This invention relates to magnetic recording media utilizing particulate magnetic material, and in particular to media having both increased concentrations of magnetic particles and a preferred particle orientation.
The invention, as well as the prior art, will be described with reference to the figures of which:
FIGS. 1, 2, 3, 4 are illustrations of geometrical concepts useful in understanding both the prior art and the invention,
FIG. 5 is a schematic representation of longitudinal magnetic recording known in the prior art,
FIGS. 6, 7 are schematic representations of longitudinal magnetic disk recording known in the prior art,
FIG. 8 is an illustration of perpendicular magnetic recording known in the prior art,
FIGS. 9a, 9b, 10a, 10b are illustrations useful in understanding one purpose of the invention,
FIGS. 11, 12 are illustrations of the electrodeposition of magnetic coating according to the prior art,
FIG. 13 is a graph useful in understanding the principle of the invention,
FIGS. 14a, 14b are illustrations of particle alignment useful in understanding the invention, and
FIG. 15 is a block diagram of electrodeposition apparatus according to the invention.
2. Description Relative to the Prior Art
Attention is initially directed to U.S. Pat. No. 4,578,280 issued Mar. 25, 1986 to Greiner et al, which discloses state of the art magnetic media useful for perpendicular magnetic recording, and to U.S. Pat. No. 4,585,535 issued Apr. 29, 1986 to Sher et al which discloses an electrophoretic method of producing magnetic recording media.
The continuing trend in digital magnetic recording is towards increased data density recorded on the magnetic medium. This requirement translates into both the recording of more tracks per inch and the recording of shorter signal wavelengths, resulting in higher areal data packing densities. Additionally, in modern digital recording, it is necessary that the media accommodate the capability to overwrite previously recorded data; that is, new data recorded over old data must effectively erase the previously recorded data. Overwrite is a complex phenomenon, but it is known in the art that the thinner the magnetic medium the more effective the overwriting of previously recorded long wavelengths by newly recorded short wavelengths. The data recording signals, based on the particular digital encoding method employed, generally have frequency components extending over several octaves. The overwrite (i.e. erase) capability must extend, therefore, over the entire frequency spectrum of the recorded signal. Inadequate erasure of the long wavelength signal results in a remanent long wavelength magnetization of the medium which interacts with the newly recorded signals causing unwanted pulse crowding and peak shift. A solution to this problem has been the reduction of the physical thickness of the recording medium with the goal of restricting the medium thickness of depths that can be adequately overwritten by a short wavelength signal. A price is paid, however, for this solution to the problem. The signal amplitude read from the medium during playback is proportional to the magnetization of the medium, which in turn is proportional to the total volume of magnetized material. By restricting the thickness of the magnetic coating the volume of magnetizable material is reduced, and the signal amplitude is degraded.
A variety of techniques have been employed in the prior art to address the volume of magnetizable material problem. In the production of media, the most common fabrication technique involves the coating of particulate magnetic material in suspension with a binder and solvent onto a substrate material. The solvent is driven off during a drying process and the medium is then compacted or "calendered" to increase the density of the magnetic layer. The final density attained is, to a large degree, dependent upon two factors: First, because the coating process involves the flow of the particle-binder-solvent mixture onto the substrate, rheological parameters controlling the dynamics of this flow establish the minimal amount of solvent needed for stable flow. When the solvent is driven off during drying, voids remain; the more solvent utilized, the greater the proportion of voids. While calendering reduces the effects of the voids, the calendering does not compact the material so as to completely eliminate the effects of the voids. Second, the geometric form of the particle itself sets a limit on the density which may be attained even if the compacting were 100 percent efficient in removing the voids. For example, considering a dispersion of identical spherical particles, the maximum packing fraction [the total volume of the spheres divided by the total volume of a cube enclosing them, expressed either as a decimal or percentage] has a theoretical maximum of 0.72. No amount of calendering can compact a medium consisting of identical spherical particles to a greater density.
FIG. 1 shows the hexagonal close-packed structure of spherical particles, and the inevitable typical interstices 10, 12, 14 which give rise to the above mentioned packing fraction limitation of 0.72. Generally, magnetic recording particles are not spherical but are needle-like, or acicular, in shape. Such particles typically have aspect ratios, i.e. the ratio of the length of the major axis to that of a minor axis of the particle, ranging from two to seven. For simplicity, it is advantageous to consider acicular particles of ideal cross section, i.e. uniform cross sections which are either rectangles or circles for the entire length of the particle. Considering the ideal case of acicular particles having a rectangular cross section and aligned as shown in FIG. 2, the resultant structure has a packing fraction of 1.0. If particles of cylindrical cross section are randomly arranged in a coating, large interstices, 13, 15 [FIG. 3] will inevitably occur with attendantly reduced packing fraction. On the other hand, if such particles were perfectly aligned as shown in FIG. 4, they would have a packing fraction of 0.91 due to the typical interstices 16, 18, 20 arising from the circular cross sections. While actual particles approximate the aforementioned shapes, in reality, magnetic particles tend to be cigar-shaped and of irregular cross section. In practice, considerably lower packing fractions, of the order of 0.4, are realized in prior art fabrication of coated magnetic tapes and disks; the reasons being the presence of voids, the fact the particles are actually cigar-shaped, and the problem of not being able to completely align the particles.
It is desirable, at this point, to describe briefly the technique known in the art for measuring the packing fraction of particulate magnetic madia. The saturation magnetization of a known volume of the magnetic medium is measured by means of a vibrating sample magnetometer. The saturation magnetization is directly proportional to the volume of magnetizable material in the sample; the greater the volume of megnetizable particles relative to the volume of binder or voids (such as those due to particle misalignment), the higher the measured saturation magnetization. Data is also available in the art for the "intrinsic" saturation magnetization values for the magnetic material under consideration; that is, the saturation magnetization of the material, not in particle form, but as a continuous atomic crystalline structure. This may be visualized as a homogeneous bar of the basic magnetic material; it is the most dense configuration in which the given magnetic material could exist. The ratio of the saturation magnetization per unit volume of the sample to the intrinsic saturation magnetization of the material is defined as the packing fraction of the particulate medium, and it will be appreciated that the packing fraction is a definitive measure of the density of recordable material present in the medium.
The desirability of alignment of acicular particles in the medium relates both to the resultant increased density of particles, and also to the parameters of the recording process itself. Magnetic particles exhibit "shape anisotropy," that is, the preference for particle magnetization to occur along a particular geometric dimension of the particle. An acicular particle generally sustains magnetization along the particle's long or major axis, and therefore the particle's orientation directly impacts the effectiveness of the recording process. The relation between particle orientation and the recording process may be appreciated by consideration of the most common technique of recording . . . longitudinal recording. In FIG. 5, longitudinal recording is performed by a recording head 22 which is in contact with a moving magnetic medium 24. A magnetic field 26, generated at the gap 28 of the head 22, is applied longitudinally with respect to the medium 24. The field 26 magnetizes the particles in the medium 24 in the direction of the field with a resultant remanent magnetization 30 in the medium. When current in the winding 32 of the head 22 reverses direction, the field 26 reverses as does the direction of magnetization 34 in the medium. In view of the previously described particle recording characteristic, the fabrication of magnetic media for use in longitudinal recording mandates that all the acicular particles lie essentially parallel to the direction of the longitudinal field. It will be appreciated this also conforms to the geometric requirement for increasing the density of magnetic material in the coating by improving the packing fraction. To accomplish this during tape manufacturing a magnetic field is applied to the coating to align the particles before the coating is dried. [Jorgensen, F., "The Complete Handbook of Magnetic Recording", Blue Ridge Summit, Pa.: Tab Books, 1980, p. 38]. Under the action of the field, the acicular particles rotate against the resisting couple due to the viscosity of the binder-solvent mixture until their major axes are aligned with the direction of the applied field. Even with the application of magnetic fields, however, the packing fractions attained are approximately only one half the theoretical values due to the appreciable viscosity of the coating mixtures, and the voids and deviations from ideal particle shapes previously described.
In the manufacture of magnetic webs for the fabrication of magnetic floppy disks, the goal of increased packing density is further limited by the shape anisotropy of the typical acicular particle. It will be appreciated that if the above described procedure was followed in fabrication a web of magnetic material for disk application, the resultant disk would have all the particles aligned in one direction. Such a disk, 36, for use in longitudinal digital recording is illustrated in FIG. 6. The arrows 35 show the direction of alignment of the major axes of the particles, which, as previously stated, is also the preferred direction of magnetization due to shape anisotropy. The longitudinally oriented recording field 38 of a record/playback head 34 operating on the disk 36, would sometimes be aligned with the particles' major axes, [FIG. 6], and one quarter of a disk rotational cycle later [FIG. 7] would be oriented perpendicular to the particles' major axes. The result is a continually varying orientation between the head field direction and the particle axes as the disk rotates. This produces a variation in magnetization, causing a "twice around" variation in the amplitude and phase of the recorded playback signal. To avoid the particle alignment responsible for this unwanted signal modulation, the particles are subjected to an intense a.c. magnetic field as they are coated onto the web substrate. This field randomizes the directions of the axes of the particles and obviates the "twice around" problem. The penalty, however, is that the disk has a relatively low packing fraction due to the resultant random orientation of the particles, and attendantly, the density and available signal are reduced. The relative magnetic moment in the desired direction is also only about half that of the aligned particle value, further reducing the signal.
It is to be noted that the previous discussion of longitudinal recording and its associated media specifies that the recording field is parallel to the plane of the medium and that the particles are aligned in the plane of the medium. In the magnetic recording art, an area of intense current interest is that of "perpendicular" recording where, by way of contrast, the recording field is perpendicular to the plane of the medium. Perpendicular recording is illustrated in FIG. 8, where a single pole head 48 is positioned above a medium 50 having the ability to support magnetization in a direction perpendicular to the plane of the medium 50. Current through the winding 53 of the head 48 generates a magnetic field 56 which penetrates the medium 50 perpendicular to its plane and magnetizes the medium 50 in the field direction. The remanent magnetization 52 is thus perpendicular to the plane of the medium. In the prior art, particulate perpendicular recording media has generally not been available, and perpendicular recording has been restricted to such practices as those using cobalt-chromium alloys sputtered onto a substrate. It is known that such alloys exhibit vertical anisotropy, and current perpendicular recording has been virtually confined to the use of such alloys. The recognized problem of providing a "particulate medium for perpendicular recording" is referred to by White, Robert M. (Editor), "Introduction to Magnetic Recording", New York, N.Y.; IEEE Press, 1985, p. 69 wherein it is stated:
S. Iwasaki of Tohoku University has suggested that data might be stored perpendicular to the plane of the disk. Here the end of the core of a head, and confronts the disk and throws field lines deep into the magnetic medium. As a result the data are stored, so to speak, on end. More than 100,000 magnetic reversals per inch might be possible, but the implementation awaits the development of both the head and the medium for it. By the technique described above the magnetic dipoles of iron oxide can be oriented in the plane of the disk. The problem is to find a way in which such dipoles can be oriented perpendicular to the plane of the disk.
As will appear below, the present invention teaches how to so orient the particles and, unlike the processes disclosed in U.S. Pat. Nos. 4,578,280 and 4,585,535, does not require an auxiliary magnetic field to align the particles perpendicular to the substrate.
The impetus to the application of perpendicular recording may be appreciated by reference to FIGS. 9a, 9b, 10a, 10b. FIG. 9a illustrates a longitudinally recorded transition 54. When a transition from one magnetic orientation to another occurs, the longitudinally aligned recorded cells interact in the manner of small bar magnets as illustrated in FIG. 9b. In this orientation, the bar magnets tend to demagnetize each other because each magnet's field bucks that of the other. The demagnetization effect is more pronounced for short magnets, or equivalently, in a recorded medium, for short recorded wavelengths [Jorgensen, supra p. 52]. On the other hand, in perpendicular recording, as illustrated in FIG. 10a, the fields of adjacent recorded transitions 55 interact in the manner of bar magnets as shown in FIG. 10b. In this case the fields of the bar magnets aid rather than oppose each other. For this reason perpendicular recording is of great interest in short wavelength recording technology where the demagnetizing effect present in longitudinal recording is most manifest. Currently available longitudinal recording systems attain a recording density of approximately 10,000 flux reversals per inch, while it is anticipated, as previously mentioned, that by use of perpendicular recording more than 100,000 flux reversals per inch may be possible.
In addition to the previously described web coating process, it is also known in the prior art to produce magnetic coatings by electrodeposition. This technique is an application of the method utilizing the phenomenon of "electrophoresis" which is exploited in the broad field of electropainting. Electropainting has been applied to a wide variety of painting requirements, from coating the insides of metal cans to painting automobile bodies. [Yeates, R. L. "Electropainting" Teddington, Great Britain: Robert Draper, Ltd., 1966]. In Japanese Patent Publication Number 25321/1977 entitled "Electrodeposition Paint for Magnetic Recording", a mixture of magnetic particles in a water soluble acyrilic polycarbonate resin emulsified uniformly in water is described for use in the electrodeposition of a mgnetic coating.
The Japanese technique is illustrated in FIG. 11. A vessel 42 contains a mixture 43 of solvent, binder and magnetic particles. A cathode 44 and an anode 46 are immersed in the mixture 43, and a d.c. voltage source 48 is connected between the cathode 44 and the anode 46. Each particle tends to acquire an electric charge due to the interface between the particle and the binder-solvent solution. A diffuse layer of charge then surrounds the charged particle, so that the combination of charged particle and the diffuse layer is electrically neutral. The particle moves under the electric force due to the field between the cathode and anode, and the diffuse layer tends to move with the particle to which it is attached but, at the same time, the diffuse layer is electrically attracted to the other electrode. As the particle moves, new charge builds in front of the particle in the direction of motion, and the diffuse layer dissipates in the opposite direction. [Bockris, J. M., and Reddy, A. K. N. "Modern Electrochemistry", New York: Plenum Press, 1974, Vol. 2, p. 833]. In the previously referenced Japanese Patent Publication, the electric field magnitude is indicated to be approximately 17 volts/cm.: a field sufficient to move the magnetic particles to the electrode where they deposit as a magnetic coating 41 [FIG. 12].